1. Field of the Invention
This invention relates to a light dimming system for use in, for example, a theatre, or a television or film studio. This invention relates also to a dimmer suitable for, but not limited to, use in such a system.
2. Description of the Related Art
Television and film studios require high power light sources (often mobile) for selective illumination of particular characters, as do live entertainment venues such as theatres. It is necessary to be able to control the level of illumination, both to provide a constant predetermined level and for providing fade-in and fade-out effects. The control circuit for controlling the level of illumination is known as the "dimmer".
In the past, dimmers using autotransformers that is, transformers with a sweep connector to vary the number of turns on the secondary coil have been used. Typically, the AC mains supply would be connected to the primary coils of a bank of such transformers, each secondary coil being individually connected to a separate light. However, each transformer occupied considerable space and was cumbersome to use.
Alternative dimmer structures used variable resistors (reostats) which are smaller, but these generate considerable heat because of the high current through the resistance of the coils. A bank of such reostats therefore generated a large amount of heat, leading to the need for cooling, and to potential mechanical reliability problems.
In recent years, it has become known to use phase control as a method of controlling AC power. In phase control systems, a semiconductor switch switches the AC supply voltage on once each half-cycle. By controlling the timing at which the semiconductor element switches on, the proportion of the mains cycle gated through the semiconductor element, and hence the average power of the output, is controlled. The components of the phase control dimmer are relatively cheap, and do not involve mechanical movement. Further, the phase controlled dimmer is controllable from a low voltage source such as the output of a micro-computer, making it possible to digitally control the light level. Phase control dimmers have found application in power control in general, and in most lighting control applications. However, major problems arise in some applications. Because the semiconductor switch gates the AC mains supply voltage in a series of sharp steps, much of the power in the output is contained in harmonics of the mains frequency. The magnitude of the harmonics varies with the time of switching of the semiconductor switch (and hence the level of light) and the rate of rise of current flow (or the switching speed). The harmonics manifest themselves in two undesirable effects.
Firstly, electromagnetic interference is generated across a broad spectrum. Higher frequency components will be radiated outwardly of the dimmer and the load circuit, and could interfere with other electrical equipment nearby.
Secondly, where the lamp is an incandescent filament lamp, magnetomotive forces within the filament are caused, generating mechanical vibrations at harmonics of the mains frequency, and these mechanical vibrations are audible as "lamp sing"; they can also reduce the life of the filament.
Several approaches to reducing the magnitude of these effects have been proposed. In U.S. Pat. No. 3,691,404 and 4,287,468, and in a product marketed by Colortran, a dimmer is provided in which the AC cycle is switched once each cycle, rather than once each half-cycle. This reduces the magnitude of such effects, although it does not eliminate them. However, it leads inevitably to the introduction of a DC component in the output current, and can lead to the supersition of a DC voltage back onto the mains supply, which is unacceptable in many applications. Usually the rate of change in current flow is controlled by connection of a series inductor within the load circuit.
An alternative approach is proposed in U.S. Pat. No. 4,633,161 (equivalent to WO 86/01365, EP 85904329.1). In this approach, the operation of the semiconductor switch is controlled to provide a slow transition rather than a sharp step. This reduces the magnitude of the higher harmonics. However, during the slow transition the semiconductor switch dissipates considerable heat and consequently this approach is limited to approximately the same rate of change of current that a large inductor can achieve, unless large and expensive semiconductors switches and cooling means are used.
It is stated in U.S. Pat. No. 4,633,161 that the application of DC switched mode power supply operation (described for example in U.S. Pat. No. 4,450,384 and 4,885,508) to the control of incandescent lamp loads operating on AC services is obvious. However, no such application is known to have been attempted. Furthermore, U.S. Pat. No. 4,633,161 teaches that such a dimmer would be prohibitively expensive and unreliable.
There is therefore, as yet, no known or available solid-state dimmer which does not suffer to some extent from lamp sing when used with an incandescent lamp, or from excessive harmonic generation. In a live performance, these drawbacks may go unnoticed as there is generally a level of ambient noise present. However, in a studio this is often far from being the case. Firstly, studio lights are often close to the subjects of illumination. The acoustic noise from the lamp intrudes into the sound field of the subject and will be picked up by the microphone since sound stages have low ambient noise levels. In fact, microphones must often be placed close to the luminaires, generally suspended above the subject, to keep them out of sight of the cameras. This naturally increases the acoustic noise reaching the microphone. Further, the optical system associated with floodlights and scoop luminaires also acts to focus the lamp sing and direct it towards the subject, and the lamp housing itself often sympathetically vibrates and acts as a sounding board.
Further, as noted in "Lamp Acoustical Noise and the Reverse Phase Control Dimmer", Burkhart and Burtness, IEEE Trans. on Industry Applications, Vol.1A-8, No.1, January/February 1972, there is an increasing interest in placing the dimmers individually at the lamps which they dim, so as to reduce the heat problems associated with dimmer banks and to simplify the power distribution cabling. This industry trend is disclosed also in U.S. Pat. Nos. 4,366,161, 4,095,139 and U.S. Pat. No. 4,057,751. However, in a studio where the lamps are suspended from or near the ceiling, the dimmers will inevitably be close to other wiring passing at ceiling level and this may, as discussed above, include sound cables so that in addition to acoustic noise, the sound channels may be corrupted by electrical by coupled noise.
There is thus a need for a studio dimming system, for supplying power to incandescent lights from the AC mains, which can be used on sound stages without electrical and acoustic noise. SUMMARY OF THE INVENTION
Accordingly, in one aspect, the present invention provides a studio dimming system which comprises an AC mains power point, power distribution means coupled to the AC mains, a dimmer assembly comprising a plurality of co-located dimmer circuits each coupled to the distribution means to receive AC power therefrom, and each comprising an output port developing an AC power signal of frequency corresponding to the AC mains, each said dimmer circuit comprising a current path between the distribution means and the output port, a switching means located in the current path so as to controllably interrupt passage of current, switch driving means supplying a control signal to the switching means to cause it to interrupt the current path a multiplicity of times within each AC half cycle, and filtering means in the current path between the switching means and the output port arranged to filter the current therethrough, the filtering means being arranged to attenuate current components at the frequency of operation of the switching means, and not substantially to attenuate current components at the frequency of the mains, the switch driving means being controllable to vary the duty factor of operation of the switching means, so as to provide at the output port a power signal at the mains frequency with an RMS voltage amplitude related to the duty factor of the switching means; dimmer control means connected via a communication link to the switch driving means of each dimmer circuit to vary the duty factor thereof in accordance with a desired power output thereof; the output port being provided to connect to a power cable running to one or more luminaire units to be supplied with power from a respective dimmer unit.
Furthermore, there is a need for an improved AC power control circuit (for example for use as a dimmer) usable in such an application. Accordingly, in another aspect the present invention provides a dimmer circuit comprising AC input terminals, AC output terminals, switching means arranged to interrupt the AC current path between the input terminals and the output terminals a multiplicity of times throughout an entire AC mains cycle, and switch driving means arranged to vary the average time for which the current path is interrupted, within each AC half cycle, and filtering means arranged to filter out current fluctuations of frequencies above the AC frequency, the variation of the average interruption time of the switching means being such as to change the shape of the AC voltage output waveform relative to the AC input waveform.
In one embodiment, the output wave form is changed to resemble the traditional prior art forward or reverse phase control wave form, in which the AC supply is gated off for a portion of each half cycle and on for the other portion, but with the effect of the relatively slow power transitions described in W086-01365, but avoiding the need for the substantial heat dissipation in the switch of the apparatus of W086-01365. In particularly preferred embodiments, the generation of harmonics is further reduced by avoiding second order discontinuities in the output voltage (i.e. abrupt changes of slope of voltage over time).
In another aspect, the invention provides a forward or reverse phase control dimmer with a relatively long transition in output power so as to reduce harmonic in generation, which the output wave form is controlled to avoid second derivative discontinuities.
In the described embodiments, forward phase control is preferred where the load to be supplied is inductive in nature, whereas reverse phase control is generally preferred where the load is predominantly resistive.
Other aspects of the invention are as described or claimed hereafter.